Method of Providing a Descriptor for at Least One Feature of an Image and Method of Matching Features

ABSTRACT

A method of providing a descriptor for at least one feature of an image comprises the steps of providing an image captured by a capturing device and extracting at least one feature from the image, and assigning a descriptor to the at least one feature, the descriptor depending on at least one parameter which is indicative of an orientation, wherein the at least one parameter is determined from the orientation of the capturing device measured by a tracking system. The invention also relates to a method of matching features of two or more images.

This application is a continuation of U.S. patent application Ser. No. 14/567,668 filed Dec. 11, 2014, which is a continuation of U.S. patent application Ser. No. 13/502,952 filed May 14, 2012, now U.S. Pat. No. 8,942,418, which is a national stage application of PCT Application No. PCT/EP2010/057461 filed on May 28, 2010, which claims priority to German Application No. 10 2009 049 849.4 filed Oct. 19, 2009.

BACKGROUND 1. Technical Field

The present invention relates to a method of providing a descriptor for at least one feature of an image and to a method of matching features of two or more images. Moreover, the invention relates to a computer program product comprising software code sections for implementing the method according to the invention.

2. Background Information

Many applications in the field of computer vision require finding corresponding points or other features in two or more images of the same scene or object under varying viewpoints, possibly with changes in illumination and capturing hardware used. The features can be points, or a set of points (lines, segments, regions in the image or simply a group of pixels). Example applications include narrow and wide-baseline stereo matching, camera pose estimation, image retrieval, object recognition, and visual search.

For example, Augmented Reality Systems permit the superposition of computer-generated virtual information with visual impressions of a real environment. To this end, the visual impressions of the real world, for example captured by a camera in one or more images, are mixed with virtual information, e.g., by means of a display device which displays the respective image augmented with the virtual information to a user. Spatial registration of virtual information and the real world requires the computation of the camera pose (position and orientation) that is usually based on feature correspondences.

A common way, e.g. such as described in David G. Lowe: “Distinctive Image Features from Scale-Invariant Keypoints”, International Journal of Computer Vision, 60, 2 (2004), pp. 91-110, to gain such correspondences is to first extract features or interest points (e.g. at edges, corners or local extrema) from the individual images that have a high repeatability. That is, the probability that the same sets of pixels corresponding to the same physical entities are extracted in different images is high. The second step is then to create a descriptor for each feature, based on the intensities of its neighborhood pixels, that enables the comparison and therefore matching of features. The two main requirements for a good descriptor are distinctiveness, i.e. different feature points result in different descriptors, and invariance to

1) changes in viewing direction, rotation and scale,

2) changes in illumination,

image noise.

This is to ensure that the same feature in different images will be described in a similar way with respect to a similarity measure. To address the invariance against rotation, a spatial normalization transforms the pixels of the local neighborhood around a feature point to a normalized coordinate system prior to the construction of the descriptor.

It is critical to the invariance that this normalization is reproducible. More advanced methods exist, but in the simplest case the normalization only consists of an in-plane rotation according to the feature orientation. The orientation is usually defined based on the pixel intensities in the neighborhood of a feature point, e.g. as the direction of the largest gradient. Ideally the pixels in the normalized neighborhood of a feature are identical for different images taken with varying viewing direction, rotation and scale. In practice, they are at least very similar, cf. FIG. 2.

In FIG. 2, there is shown an exemplary feature point in different scenes 21 and 22. In the first column showing the scenes 21 and 22, the same feature point under two different orientations is shown as feature point F21 in scene 21 and feature point F22 in scene 22. In a next step the orientation is defined based on the pixel intensities in the neighborhood of the respective feature point F21 and F22, in the present example as the direction of the largest gradient (depicted by the white line within the respective rectangle). Then, a spatial normalization transforms the pixels of the local neighborhood around feature points F21 and F22 (in the present case, the pixels within the rectangle) to a normalized coordinate system (depictions 31 and 32 in the second column) prior to the construction of the descriptors d1 and d2 (third column), respectively. As a result, alignment of the descriptors d1, d2 to the largest gradient results in a very similar normalized neighborhood (as shown in depictions 31 and 32) and, therefore, similar descriptors d1 and d2. This property is common among local feature descriptors and referred to as invariance to rotation. Invariance to scale is usually handled by constructing an image pyramid containing the image at different scales and performing the above on every scale level. Other approaches store the scale with every feature descriptor.

A variety of local feature descriptors exist, wherein a good overview and comparison is given in Krystian Mikolajczyk and Cordelia Schmid, “A performance evaluation of local descriptors”, IEEE Transactions on Pattern Analysis & Machine Intelligence, 10, 27 (2005), pp. 1615-1630. Most of them are based on the creation of histograms of either intensity values of the normalized local neighborhood pixels or of functions of them, such as gradients. The final descriptor is expressed as an n-dimensional vector (as shown in FIG. 2 on the right) and can be compared to other descriptors using a similarity measure such as the Euclidian distance.

In FIG. 3, there is shown a standard approach for creating a feature descriptor. In step SI, an image is captured by a capturing device, e.g. a camera, or loaded from a storage medium. In step S2, feature points are extracted from the image and stored in a 2-dimensional description (parameters u, v). In step S3, an orientation assignment is performed as described above with respect to FIG. 2, to add to the parameters u, v an orientation angle a. Thereafter, a neighborhood normalization step S4 is performed, as described above with respect to FIG. 2 to gain normalized neighborhood pixel intensities i[ ]. In the final step S5, a feature descriptor in the form of a descriptor vector d[ ] is created for the respective extracted feature as a function of the normalized neighborhood pixel intensities i[ ]. Approaches exist that may assign multiple orientation angles to a feature in step S3 and consequently carry out the steps S4 and S5 for each orientation resulting in one descriptor per assigned orientation.

A major limitation of the standard approaches as described above is that while invariance to rotation is clearly an important characteristic of local feature descriptors in many applications, it may however lead to mismatches when images contain multiple congruent or near-congruent features, as for instance the four corners of a symmetric window or individual dartboard sections.

In an example, as shown in FIG. 1, a real object 3 which is in the present example a building having a window 4, is captured by a mobile device 1 having a camera on the rear side (not shown). For instance, the mobile device 1 may be a mobile phone having a camera and an optical lens on the rear side for capturing an image of the window 4. On the display 6 of the mobile device 1, the window 4 is depicted as shown. An image processing method extracts features from the displayed image, for example the features F1 to F4 representing the four corners of the window that can be considered as prominent features of the window, and creates a feature descriptor for each of the features F1 to F4. Due to invariance to rotation, as schematically illustrated in FIG. 1 in the left column, an ideal local feature descriptor would describe these features F1 to F4 in exactly the same way making them indistinguishable, as illustrated by the extracted features F1 to F4 depicted in a normalized coordinate system in the left column.

In a real word setting with camera noise and aliasing, the descriptors will not be identical but very similar and therefore virtually indistinguishable. Consequently, the probability of mismatches is very high for such scenes which may result in a complete failure of any system relying upon such local feature descriptors.

A variety of approaches exist that assume all camera images to be taken in an upright orientation and therefore do not need to deal with the orientation. Here congruent or near-congruent features in different orientations can easily be distinguished from each other, but the field of possible applications is very limited since the camera orientation is heavily constraint.

Therefore, it would be beneficial to have a method of providing a descriptor for at least one feature of an image, wherein the descriptor is provided in a way that the probability of mismatches due to congruent or near-congruent features in different orientations on a static object or scene in a feature matching process may be reduced without constraining the orientation or movement of the capturing device or without needing prior knowledge on the orientation or movement of the capturing device.

SUMMARY

In a first aspect, there is provided a method of providing a descriptor for at least one feature of an image, the method including the steps of providing an image captured by a capturing device and extracting at least one feature from the image; and assigning a descriptor to the at least one feature, the descriptor depending on at least one parameter which is indicative of an orientation, where the at least one parameter is determined from at least one tracking system. Further, in another aspect, there is provided a method of matching features of two or more images, the method including extracting at least one first feature of a first image and at least one second feature of a second image; providing a first descriptor for the first feature and a second descriptor for the second feature, where at least one of the first and second descriptors is provided according to the method of providing a descriptor for at least one feature of an image described above; and comparing the first and second descriptors in a matching process for the first and second features. The invention is also concerned with a computer program product adapted to be loaded into the internal memory of a digital computer system coupled with at least one capturing device for capturing an image or a memory medium for loading an image from, and comprising software code sections for implementing such methods when said product is running on said computer system.

Particularly, in a first aspect, there is provided a method of providing a descriptor for at least one feature of an image, comprising the steps of providing an image captured by a capturing device and extracting at least one feature from the image, and assigning a descriptor to the at least one feature, the descriptor depending on at least one parameter which is indicative of an orientation, wherein the at least one parameter is determined from an absolute or relative orientation of the capturing device measured by a tracking system. Particularly, the tracking system in the context of this invention determines at least one orientation of an object, in particular of the capturing device, preferably with regard to a common coordinate system, as further described in the embodiments below. With the orientation being measured by a tracking system, it is not necessary to constraint the orientation and/or movement of the capturing device to a certain position or to have any prior knowledge on the orientation and/or movement of the capturing device.

In an aspect of the invention, it is proposed to align the orientation of feature descriptors (particularly local feature descriptors) with a certain given common coordinate system. Instead of gaining a reproducible orientation from the intensities of neighboring pixels in the image, additional information on the orientation of individual pixels and/or orientation of the capturing device builds the basis for orientation assignment to a feature. Aligning local feature descriptors to a global orientation overcomes ambiguities resulting from congruent or near-congruent features with different orientations as they are widespread in urban scenes and on man-made objects, as illustrated in FIG. 1 on the right and described in more detail below. In an embodiment of the invention, the tracking system comprises a sensor attached to the capturing device such as an inertial sensor, an accelerometer, a gyroscope, or a compass.

In another embodiment, the tracking system comprises a mechanical tracking system based on a physical connection between the capturing device and a fixed reference point, an electromagnetic tracking system where magnetic fields are generated and measured, an acoustic tracking system working with acoustic waves, and/or an optical tracking system using light emitted and/or reflected from the capturing device. Said optical tracking system can be either integrated into the capturing device or be realized as an external system separately from the capturing device.

In one embodiment of the invention, the common coordinate system is a world coordinate system as measured with a tracking system delivering absolute values (e.g. compass and/or inertial sensors attached to the device).

In another embodiment, the common coordinate system comprises any coordinate system relative to which a tracking system delivers transformations. Relative changes in orientation, for instance measured with a gyroscope, can be accumulated to compute the absolute orientation in common coordinates at every instant without having any sensor that measures the absolute orientation.

In another embodiment, the capturing device comprises a range data capturing device, particularly a laser scanner, time-of-flight camera, or a stereo camera, which provides image pixels with an associated depth and/or 3D position.

In another aspect of the invention, the method further comprises the step of normalizing the neighborhood of the at least one feature with respect to the orientation in a common coordinate system. Particularly, in the step of normalizing the neighborhood of the at least one feature the orientation provides an angle for rotating the image. Optionally, in the step of normalizing the neighborhood of the at least one feature the orientation is used to warp the neighborhood pixels or the entire image to one or more reference orientations to correct for perspective distortions in particular feature neighborhoods.

In another aspect of the invention, a method of matching features of two or more images comprises the steps of extracting at least one first feature of a first image and at least one second feature of a second image, providing a first descriptor for the first feature and a second descriptor for the second feature, wherein at least one of the first and second descriptors is provided according to aspects of the method as described above, and comparing the first and second descriptors in a matching process for the first and second features. In the matching process it may then be determined based on a similarity measure whether the first and second features correspond with each other.

According to an embodiment of the invention, one or more directions of the at least one feature are computed based on pixel intensities of neighboring pixels and stored with respect to the common coordinate system. In the matching stage only features with similar directions with respect to the common coordinate system are matched to reduce the number of comparisons needed and decrease the ratio of false matches.

According to embodiments of the invention, the method may be implemented in a process of stereo matching, particularly wide-baseline stereo matching, camera tracking, image retrieval, object recognition, visual search, pose estimation, visual surveillance, scene reconstruction, motion estimation, panorama stitching or image restoration.

In a further aspect of the invention, there is provided a computer program product adapted to be loaded into the internal memory of a digital computer system coupled with at least one capturing device for capturing an image, and comprising software code sections by means of which the steps according to any of the methods and embodiments as described herein are performed when said product is running on said computer system.

Further embodiments and aspects of the invention will be apparent from the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained in more detail with reference to the following figures in which aspects of the invention are depicted according to various exemplary embodiments.

FIG. 1 shows an exemplary scene in which an image is captured by a mobile device having a camera on the rear side thereof, and in which features are extracted from the image and schematically shown in a normalized depiction according to a standard approach (left) and according to an embodiment of the invention (right),

FIG. 2 shows a standard approach to invariance to rotation in an exemplary depiction of a feature point under two different orientations, a normalization of local neighboring pixels of the feature points and the construction of a respective descriptor,

FIG. 3 shows in a flow diagram a standard approach for constructing a feature descriptor,

FIG. 4 shows in a flow diagram an embodiment of a method according to the invention for constructing a feature descriptor,

FIG. 5 shows in a schematic manner an exemplary first image and second image depicting the same static object from different viewpoints with respective extracted features which are to be matched.

DETAILED DESCRIPTION

FIG. 1 shows an exemplary scene in which an image is captured by a mobile device having a camera on the rear side thereof, and in which features are extracted from the image and schematically shown in a normalized depiction according to a standard approach (shown on the left and as described above) and according to an embodiment of the invention (shown on the right).

According to the shown embodiment of the invention, again the real object 3, which is in the present example a building having a window 4, is captured by a mobile device 2 having a camera on the rear side (not shown). For instance, the mobile device 2 may be a mobile phone or digital camera having a microprocessor for image processing and a camera with an optical lens on the rear side for capturing an image of the window 4. However, any other type of device may be used. Likewise, any other type of system configuration with a processing device containing a microprocessor for image processing and a camera may be used either in integrated or distributed form.

The mobile device comprises a sensor 5, for example an inertial sensor, an accelerometer, a gyrometer and/or a compass. As such, the sensor is associated with the camera of the mobile device as the camera is also part of the mobile device. The sensor is appropriate for measuring an orientation of the mobile device 2 with reference to a common coordinate system 10. This can be measured either in an absolute manner or by accumulating relative orientation data over time. Commonly available mobile phones and digital cameras are often equipped with built-in digital accelerometers and compasses that provide a measured value indicative of the current orientation of the device. This information, along with in-formation about the intrinsic parameters of the camera enable the transformation of any orientation in world coordinates, e.g. of the gravitational force or the north, in the coordinate system of the camera image.

On the display 7 of the mobile device 2, the window 4 as captured by the camera of the mobile device 2 is depicted as shown. An image processing method performed in a microprocessor of the mobile device 2 (or of an external device communicating with the mobile device 2) extracts features from the captured image, for example the features F11 to F14 representing the four corners of the window as rather prominent features of the window, and creates a feature descriptor for each of the features F11 to F14, as described in more detail below. The created local feature descriptor describes these features F11 to F14 in a different way as compared to the left column (standard approach as described above) making them clearly distinguishable, as illustrated by the extracted features F11 to F14 depicted in a normalized coordinate system in the right column. Particularly, aligning the feature orientation to a global orientation as defined by coordinate system 10 of the sensor 5 results in four well distinguishable descriptors without constraining the device orientation. Features taken under different camera orientations can be matched.

In FIG. 4, there is shown a flow diagram of an embodiment of a method according to the invention for constructing a feature descriptor. In step SI 1, an image is captured by a capturing device, e.g. the camera of the mobile device 2, or loaded from a storage medium. In step SI 2, feature points are extracted from the image to gain feature points in a 2-dimensional description (parameters u, v). In step S13 the feature orientation is computed for the extracted feature point using spatial information on the orientation of the capturing device (parameters x, y, z) provided by a tracking system.

For example, the tracking system gives the orientation of the capturing device with respect to a world coordinate system as Euler angles and feature descriptors are supposed to be aligned with the gravitational force. A very simple way to gain the orientation for all features is to transform the gravitational force to a coordinate system attached to the capturing device using the Euler angles first and then project it onto the image plane. Thereby, the direction of the gravitational force in the image is computed and used for all features in the image. This technique assumes orthogonal projection which is generally not the case. Incorporating the intrinsic parameters of the camera relaxes this assumption but still all techniques based on 2D images assume everything visible in the image to lie on a plane and therefore are approximations.

In step SI 4, an orientation assignment is performed to add to the parameters u, v an orientation angle a based on the feature orientation angle a determined in step SI 3. Thereafter, a neighborhood normalization step SI 5 is performed to gain normalized neighborhood pixel intensities i[ ]. In the final step SI 6, a feature descriptor in the form of a descriptor vector d[ ] is created for the respective extracted feature depending on a parameter which is indicative of an orientation of the extracted feature, particularly resulting from the orientation assignment in step SI 4.

According to another embodiment of the invention, as the capturing device a range data capturing device may be used, wherein pixels of images taken with any kind of range data capturing device, such as laser scanners, time-of-flight cameras, or stereo cameras may have associated 3D coordinates. In this case any orientation in a common coordinate system for a particular feature point can be computed from the 3D positions of the neighboring pixels of the feature point.

Given a feature at pixel P, for all neighborhood pixels Ni (where i is the index of the neighboring pixel), the 3D vector originating from P and pointing to Ni is computed. The two nearest neighbors to the desired orientation vector are determined and used to interpolate the desired orientation in image space.

Furthermore, the knowledge of the 3D position of at least two pixels allows for computing a rotation angle for in-plane rotation in the neighborhood normalization step. If the 3D world coordinates of three or more pixels are known, a three-dimensional transformation can be computed to warp the local neighborhood or the entire image to one or more reference orientations in the normalization step.

Feature descriptors extracted from images with associated spatial information of different kind from different tracking systems can be matched using the proposed technique.

FIG. 1 schematically compares the results of standard approaches against globally-aligned local feature descriptors determined according to the invention by the example of the four corners of a window, as shown and as described above. While the standard approach (features F1-F4 on the left) results in four identical descriptors, the global alignment in accordance with the invention leads to clearly distinctive feature descriptors (features F11-F14 on the right).

Therefore, according to aspects of the invention, when extracting local feature descriptors from images with associated information on the orientation of the capturing device with respect to a common coordinate system, it is proposed to assign the orientation of a feature based on this information. Particularly, the orientation for a feature is being aligned with a common coordinate system, projected to the image coordinate system. This enables higher distinctiveness between descriptors of congruent or near-congruent features with different orientations while allowing for free movement and rotation of the camera. The method according to the invention can be easily plugged into any existing local feature descriptor that relies on a normalized local neighborhood by taking into account the measured orientation, as shown in FIG. 4.

Optionally the measured device orientation does not only influence the feature orientation assignment in that it provides a single angle to rotate the camera image about in the neighborhood normalization step, but is also used to warp parts of or the entire image to one or more reference orientations to correct for perspective distortions in the particular feature neighborhoods in addition.

For example, the method as described above may be implemented in a process of stereo matching, particularly wide-baseline stereo matching, camera tracking, image retrieval, image classification, object classification or object recognition.

The goal of stereo matching is to reconstruct the 3D geometry of a scene or object given two images of it taken from different viewpoints. This is done by finding corresponding pixels in the two images depicting the same 3D point and computing their depths by means of triangulation.

Camera tracking describes the process of computing the pose (position and orientation) of a camera given one or more camera images. Features in the camera image are either matched against reference features with known 3D positions to compute an absolute pose or against features from the previous frame to compute the relative change in position and orientation.

Classification of images or objects assigns a given image or three-dimensional description of an object to one of n possible classes. An example would be a method that tells if a given image depicts apples or pears, even if the particular fruit in the query image has not been used for training or as a reference image. Whereas in image retrieval and object recognition for a given query the exactly matching reference image or object is searched for.

All these techniques rely on the matching of features of two or more images.

FIGS. 5A and 5B show in a schematic manner an exemplary first image and second image depicting the same object with respective extracted features which are to be matched. For example, the images may have been captured under different conditions or circumstances, such as different perspectives, light conditions, etc.

In the first image IMI, a real static object ROI as shown in FIG. 5A is captured by a camera (not shown). In the image IMI features of the real object ROI are extracted, such as shown in FIG. 5A by features F51. In a following step, descriptors may be computed for every extracted feature F51 in accordance with the method of the invention. These features F51 are then matched with features F52 extracted in the second image IM2. The second image IM2 is depicting a real object R02 which corresponds with real object ROI under a different viewpoint, wherein for the features F52 also a respective descriptor is determined. Particularly, if the descriptors of features F51 and F52 are relatively close in terms of a certain similarity measure, they are matched. For example, if every descriptor is written as a vector of numbers, when comparing two descriptors, one can use the Euclidian distance between two corresponding vectors as similarity measure.

There are three matches shown in FIG. 5 illustrated by connecting them with a dashed line. Match M51 is a correct one since the two features matched here correspond to the same physical entity, whereas matches M52 and M53 are wrong as they match features corresponding to different physical entities. In match M52 the two features have a similar (near-congruent) neighborhood in different orientations. While such mismatches are common using standard approaches, this invention aims to avoid them. The two features matched in M53 have a similar neighborhood in a similar orientation and can therefore lead to mismatches in both standard approaches and an embodiment of this invention.

The common way to compute the orientation of a feature based on pixel intensities of neighboring pixels has a high repeatability and is therefore a reliable characteristic of a feature. When aligning features with a common coordinate system as suggested in this invention, this characteristic can be used to add additional distinctiveness to the descriptors and increase matching performance. When extracting features, optionally one or more directions based on image intensities are computed and stored with respect to the common coordinate system for each feature. In the matching stage the number of comparisons needed can be reduced by comparing only features with similar directions with respect to the common coordinate system.

This detailed description has set forth some embodiments of the present invention. It is to be understood that the above description of a possible implementation is intended to be illustrative and not restrictive. Moreover, in this disclosure the terms “first”, “second”, etc., are used merely as labels, and are not intended to impose numerical requirements on their objects. Other embodiments and modifications within the scope of the claims will be apparent to those of skill in the art upon studying the above description in connection with the drawings.

While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed herein as the best mode contemplated for carrying out this invention. 

1. A method comprising: extracting a feature from an image captured by a capturing device; determining a direction of a gravitational force with respect to the feature; and assigning a descriptor to the feature based on the direction of the gravitational force.
 2. The method of claim 1, wherein determining the direction of the gravitational force with respect to the feature includes projecting the gravitational force into the image based on intrinsic parameters of the capturing device.
 3. The method of claim 2, further comprising determining an orientation of the capturing device, wherein the direction of the gravitational force with respect to the feature is determined based further on the orientation of the capturing device.
 4. The method of claim 3, further comprising receiving orientation data from an accelerometer, a gyrometer, a compass, or a combination thereof, wherein the orientation of the capturing device is determined based on the orientation data.
 5. The method of claim 4, wherein the direction of the gravitational force with respect to the feature is determined based on a three dimensional location of the feature.
 6. The method of claim 5, wherein the three dimensional location of the feature is determined based on depth information received from the capturing device.
 7. The method of claim 6, wherein the capture device includes a laser scanner, a time-of-flight camera, a stereo camera, or a combination thereof.
 8. An apparatus comprising: one or more processors; and a memory storing instructions executable by the one or more processors to: extract a feature from an image captured by a capturing device; determine a direction of a gravitational force with respect to the feature; and assign a descriptor to the feature based on the direction of the gravitational force.
 9. The apparatus of claim 8, further comprising the capturing device.
 10. The apparatus of claim 8, wherein the instructions are executable by the one or more processors to determine the direction of the gravitational force by projecting the gravitational force into the image based on intrinsic parameters of the capturing device.
 11. The apparatus of claim 8, further comprising a sensor including: an accelerometer; a gyrometer; a compass; or a combination thereof.
 12. The apparatus of claim 11, wherein the instructions are further executable by the one or more processors to: determine an orientation of the capturing device based on orientation data from the sensor; and determine the direction of the gravitational force with respect to the feature based further on the orientation of the capturing device.
 13. The apparatus of claim 8, wherein the instructions are executable by the one or more processors to determine the direction of the gravitational force with respect to the feature based on a three dimensional location of the feature.
 14. The apparatus of claim 13, further comprising a three dimensional sensor comprising a laser scanner, a time-of-flight camera, a stereo camera, or a combination thereof, wherein the instructions are further executable by the one or more processors to determine the three dimensional location of the feature based on data from the three dimensional sensor.
 15. A computer readable storage device storing instructions executable by one or more processors to: extract a feature from an image captured by a capturing device; determine a direction of a gravitational force with respect to the feature; and assign a descriptor to the feature based on the direction of the gravitational force.
 16. The computer readable storage device of claim 15, wherein the instructions are executable by the one or more processors to determine the direction of the gravitational force by projecting the gravitational force into the image based on intrinsic parameters of the capturing device.
 17. The computer readable storage device of claim 15, wherein the instructions are further executable by the one or more processors to: determine an orientation of the capturing device based on orientation data from an accelerometer, a gyrometer, a compass, or a combination thereof; and determine the direction of the gravitational force with respect to the feature based further on the orientation of the capturing device.
 18. The computer readable storage device of claim 15, wherein the instructions are executable by the one or more processors to determine the direction of the gravitational force with respect to the feature based on a three dimensional location of the feature.
 19. The computer readable storage device of claim 18, wherein the instructions are further executable by the one or more processors to determine the three dimensional location of the feature based on data from a three dimensional sensor comprising a laser scanner, a time-of-flight camera, a stereo camera, or a combination thereof.
 20. The computer readable storage device of claim 15, wherein the instructions are further executable by the one or more processors to match the feature to a second feature from a second image based on the descriptor and a second descriptor of the second feature, and wherein the second feature is based on a second direction of the gravitational force with respect to the second feature. 